Blackouts and other power inconsistencies present a problem for power users. Even seconds of downtime or minor aberrations in power quality can translate into millions of dollars of loss for businesses. The Electric Power Research Institute (EPRI) has estimated that power disturbances cost industry as much as $400 billion a year.
The public utility grid was not designed, nor is it equipped to deliver power without interruption. It also lacks the ability to modulate, condition and improve the power it delivers—increasing the risk that customers will be subjected to surges, sags and other power quality inconsistencies. Furthermore, the more than 2.5 million miles of electric wire that deliver power from the country's main grids are vulnerable to all types of risk. Severe weather can cause major outages, but even the occasional downed wire or broken pole can threaten to shut down production, leave workers idle, and/or stop communications.
Alternatives to reliance on a public utility grid include distributed generation systems that, once installed at a customer's site, can boost generation capacity for continuous and backup power, relieve transmission and distribution bottlenecks, and support power system maintenance by generating temporary backup power. Distributed power models also offer customers the flexibility to customize their power system based on their individual needs, and they are sited and installed in much less time than it takes to conduct conventional central plant system power generation upgrades.
Existing alternatives, however, still leave companies with no fully satisfactory distributed generation system. Fuel cells, for example, require more development before being suited for distributed power generation. Other options include solar, wind, reciprocating engines and micro turbines. All of these options, however, require local energy storage to work effectively. Solar and wind power are energy sources of opportunity, meaning they are not always available all day every day. Fuel cells and micro-turbines are steady state devices that can make use of natural gas. These technologies, however, do not load follow. Consequently, transients need to be supplied from storage. Use of these technologies, requires the availability of effective and reliable storage systems.
One type of energy storage system is an electrolyte battery. Such a battery can be configured as an array of stacks of cells (typically lead-acid cells), with each stack of cells having its own electrolyte. Since each stack is a closed system, the open-circuit voltage (Voc) across a stack is indicative of the amount of charge stored in that particular stack. Differences in the open-circuit voltages between stacks can be used to determine which stacks in the system are fully charged and which are only partially charged.
A second type of electrolyte battery is a flowing electrolyte battery. One such battery employs an array of stacks of cells, where the stacks share a common flowing electrolyte. Since the stacks share the electrolyte, measurements of the open-circuit voltage across a stack only indicate whether the stack stores some non-zero amount of charge, rather than indicating the stack's state of charge relative to the other stacks in the system. Moreover, differences in the open circuit voltages between stacks are typically indicative of some internal abnormality that has lowered a stack's internal resistance.
For example, in a zinc-bromide flowing electrolyte battery, the stacks share an aqueous zinc bromide electrolyte and have their own electrodes for deposit and dissolution of elemental zinc during charge and discharge cycles. In this type of battery, the electrolyte flow to a stack can be inhibited by poorly placed zinc deposits. Additionally, nucleation on the electrodes can cause dendrite formation and branching between cells. In either case, the internal resistance of the affected stack is lowered, causing a corresponding drop in the open-circuit voltage across the stack.
Differences in open-circuit voltages between stacks in flowing electrolyte battery systems can affect the charge and discharge cycles of the stacks and, potentially, the operation of the battery. For example, in the aforementioned zinc-bromide battery, a lowered open circuit voltage in a particular stack causes an increase in the rate of zinc accumulation in the faulty stack during the charge cycle and a decrease in the rate of zinc reduction in the faulty stack during the discharge cycle. Moreover, the additional zinc stored in the faulty stack typically comes from the electrolyte normally utilized by neighboring stacks. As a result of the lowered zinc availability, the energy storage capacity of the neighboring stacks may be reduced. Another consequence is that the stack having the increased zinc accumulation does not fully strip during discharge; eventually resulting in zinc accumulating on the electrodes of the faulty stack to such an extent that it causes internal short circuiting between the cells of the stack. This can potentially destroy the stack and possibly, the entire battery. A further consequence is that the increased zinc accumulation restricts the channels through which the electrolyte flows. As the electrolyte flow acts to cool the stack, the restricted flow may cause the stack to over heat and melt critical components.
Prior art solutions to this problem have involved fully “stripping” i.e., fully discharging, each stack in the battery, completely removing any stored charge from all of the cells in all of the stacks. Ideally, this process eliminates the abnormality that initially caused the difference in open-circuit voltage between the stacks. For example, a full strip typically dissolves dendrites between plates and/or deposits obstructing electrolyte flow. However, a full strip of a flowing electrolyte battery is typically time consuming (often taking one or two days to complete) and may have to be repeated every few days for a recurring problem. A full strip of the battery typically renders it unavailable or at a significantly reduced capacity for electrical applications, necessitating the purchase and installation of additional redundant battery systems. Moreover, a full strip is often unnecessary since typically a minority of the stacks in the battery is operating abnormally.
Therefore, there is a need for improved methods and apparatus for controlling, monitoring, charging and/or discharging cells in a flowing electrolyte battery.